VX-880 / zimislecel: phase 3 in T1D with a kidney transplant

What is being tested, and why it matters

Zimislecel (formerly VX-880) is Vertex's lab-grown islet cell therapy: fully differentiated insulin-producing cells, manufactured from pluripotent stem cells and infused as a single dose into the hepatic portal vein.² In the published phase 1/2 (FORWARD) results, 10 of the 12 participants with at least one year of follow-up after the full dose were insulin-independent at one year.²

It comes with one large catch. Because the cells come from a donor cell line rather than from the recipient, everyone who gets them must take immune-suppressing drugs for as long as the cells are meant to last. Those drugs carry real risks — infection, kidney damage, cancer — and for most people with type 1 diabetes that trade is the reason a working cure is still not an easy "yes".

This trial sidesteps the catch by choosing people who have already made the trade. Every participant has a kidney transplant, and is therefore already taking anti-rejection drugs to keep it. For them, the islet infusion requires no additional long-term immunosuppression beyond their existing kidney regimen.¹ It is a rare situation where the question "do the cells work?" can be asked almost on its own, separated from "are the drugs worth it?".

Who it is for

The study enrolls adults aged 18-65 who have been insulin-dependent for at least five years and who have had a single prior kidney transplant.¹ They must already be on a stable regimen — for at least four weeks — of tacrolimus plus either mycophenolate (mofetil or sodium) or sirolimus, and must be using a CGM consistently.¹ People who have had more than one kidney transplant, or any previous islet, cell, or other-organ transplant, are excluded.¹

This is a well-defined and, in one sense, well-served group: combined kidney-pancreas transplantation already exists for some of these people. What it is not is a general population of people with type 1 diabetes.

How the study is designed

NCT06832410 is an open-label phase 3 study sponsored by Vertex.¹ It is small — about 10 participants — and runs at sites in the United States (Philadelphia, Pittsburgh, Madison), Canada (Toronto, Vancouver), and Saudi Arabia (Riyadh).¹ It began recruiting on 31 March 2025.¹

The primary endpoint is simple and unusually direct: the proportion of participants who are insulin-independent one year after the infusion.¹ Estimated primary completion is 17 September 2026, with the study running to an estimated completion of 17 September 2027.¹

What to watch for

Two honest cautions. First, this is a separate study from the main pivotal trial. The FORWARD study (NCT04786262) is the larger pivotal efficacy study of zimislecel; this kidney-transplant study (about 10 participants) is a smaller companion, not the main registration trial. Second, with roughly 10 participants and no control group, it can show whether the cells engraft and free people from insulin — it cannot, on its own, settle questions of long-term durability or rare harms.

What makes it worth watching anyway is the timing and the logic. A one-year insulin-independence readout with estimated primary completion in September 2026 makes this a nearer-term datapoint among ongoing islet-replacement studies.¹ And if lab-grown islets work well in people whose immunosuppression is already a sunk cost, that strengthens the case for the parallel programmes trying to deliver the same cells without immunosuppression — the encapsulated and gene-edited versions Vertex and others are pursuing. Those programmes are where a cure for everyone else has to come from; this trial helps show what they are aiming at.